Current oxidation technologies using activated persulfate are specifically associated with applications for the treatment of organic contaminants in soils and groundwater and are limited to activation technologies using divalent iron, UV, heat, carbonate, and liquid (hydrogen) peroxide. The use of chelated divalent metal complexes prevents the second and critical step in the remedial process which is biologically mediated. These technologies are effective for the full range of organics within the saturated zone; however, each activation process targets a specific organic range of contaminants. The incorporation of a biological component in the remedial process allows for a single treatment as compared to other persulfate activation processes which requires additional oxidation events to fully treat the sorbed phases of the targeted compounds.
Chlorinated solvents and petroleum hydrocarbons, including polyaromatic hydrocarbons are compounds characterized by their toxicity to organisms at higher concentrations and are widely distributed in oil contaminated soils and groundwater.
Halogenated volatile organic compounds (VOCs), including chlorinated aliphatic hydrocarbons (CAHs), are the most frequently occurring type of contaminant in soil and groundwater at Superfund and other hazardous waste sites in the United States. The U.S. Environmental Protection Agency (EPA) estimates that cleanup of these sites will cost more than $45 billion (1996) over the next several decades.
CAHs are manmade organic compounds. They typically are manufactured from naturally occurring hydrocarbon constituents (methane, ethane, and ethene) and chlorine through various processes that substitute one or more hydrogen atoms with a chlorine atom, or selectively dechlorinate chlorinated compounds to a less chlorinated state. CAHs are used in a wide variety of applications, including uses as solvents and degreasers and in the manufacturing of raw materials. CAHs include such solvents as tetrachloroethene (PCE), trichloroethene (TCE), carbon tetrachloride (CT), chloroform (CF), and methylene chloride (MC). Historical management of wastes containing CAHs has resulted in contamination of soil and groundwater, with CAHs present at many contaminated groundwater sites in the United States. TCE is the most prevalent of those contaminants. In addition, CAHs and their degradation products, including dichloroethane (DCA), dichloroethene (DCE), and vinyl chloride (VC), tend to persist in the subsurface creating a hazard to public health and the environment.
Benzene, toluene, ethylbenzene, and xylenes (BTEX) are characterized by their toxicity to organisms at higher concentrations, and are widely distributed in oil contaminated soils, groundwater, and sediments as a result of relatively high aqueous solubility compared to other components of petroleum. As the United States Environmental Protection Agency (U.S. EPA) estimates, 35% of the U.S.'s gasoline and diesel fuel underground storage tanks (USTs) are leaking and approximately 40% of these leaking USTs likely have resulted in soil and groundwater contaminations from BTEX. BTEX are volatile and water-soluble constituents that comprise 50% of the water-soluble fraction of gasoline. The presence of BTEX in groundwater can create a hazard to public health and the environment.
BTEX are readily degradable in aerobic surface water and soil systems; however, in the subsurface environment, contamination by organic compounds often results in the complete consumption of available oxygen by indigenous microorganisms and the development of anaerobic conditions. In the absence of oxygen, degradation of BTEX can take place only with the use of alternative electron acceptors, such as nitrate, sulfate, or ferric iron, or fermentatively in combination with methanogenesis.
Polychlorinated biphenyls (PCBs) are organochlorine compounds which are mixtures of up to 209 individual chlorinated compounds referred to as congeners. These congener mixtures of chlorobiphenyl (the base chemical) are referred to by different identification systems.
PCBs have been commercially produced and sold as pure oil or in equivalent form from around 1929. They are extremely stable compounds with excellent electrical insulation and heat transfer properties. These characteristics have led to their widespread use in a variety of industrial, commercial and domestic applications.
PCBs can be released to the environment from hazardous waste sites; illegal or improper disposal of industrial wastes and consumer products; leaks from old electrical transformers containing PCBs; also incinerating some wastes. Their major disadvantage is that they do not readily break down in the environment and thus may remain there for very long periods of time. They can travel long distances in the air and be deposited in areas far away from where they were released.
While water contamination can occur, many PCBs dissolve or stick to the bottom sediments or attach themselves to organic particles. Similarly, PCBs can be easily attached to soil particles. They can also be absorbed by small organisms and fish and through the food chain can travel to other animals. PCBs accumulate in fish and marine mammals, reaching levels that may be many thousands of times higher than in water.
The U.S. EPA has established permissible levels for chemical contaminants in drinking water supplied by public water systems. These levels are called Maximum Contaminant Levels (MCLs). To derive these MCLs, the U.S. EPA uses a number of conservative assumptions, thereby ensuring adequate protection of the public. In the case of known or suspected carcinogens, such as benzene or PCE, the MCL is calculated based on assumption that the average adult weighs 154 lbs and drinks approximately 2 quarts of water per day over a lifetime (70 years). The MCL is set so that a lifetime exposure to the contaminant at the MCL concentration would result in no more than 1 to 100 (depending on the chemical) excess cases of cancer per million people exposed. The table below outlines the MCL figures for BTEX.
Chemical CompoundMCL Chemical (μg/L)PCE5TCE5Chlorobenzene100PCBs0.5Benzene5Toluene1,000Ethylbenzene700Xylenes (total)10,000
Therefore, there is a need in the art for a process of oxidation and biological attenuation of volatile and semi-volatile organic compounds, pesticides and herbicides, and other recalcitrant organic compounds in soils, sediments, clays, rocks, sands, groundwater, and all other environmental media.